This invention relates to a marine propulsion device with an improved exhaust discharge.
In marine propulsion devices such as outboard motors, stern drives and the like, it has been found that there is too much propeller area for the horsepower of the engine under certain operating conditions. One of the results is to reduce acceleration. It has previously been suggested that a large portion of the engine exhaust gases be discharged above the propeller, while the remaining small portion of the gases be discharged into the path of the upper part of the propeller to "desolidify" the water adjacent the propeller to thus, in effect, reduce the propeller area. See U.S. Pat. No. 3,745,964. In that patent, the small portion of the exhaust is entrained into the propeller by suction caused by the water passing around the gear case strut.
The arrangement of the aforementioned patent is such that only a small amount of exhaust is utilized for ventilating the propeller, with the force of this exhaust being limited by the suction forces. It is believed that the resultant improvement in acceleration is relatively minimal.
It is an object of the present invention to maximize the ventilating effect of the engine exhaust on a propeller designed for surface running and thereby maximize bringing the engine RPM up to speed. It is a further object of the invention to provide an engine exhaust arrangement which materially assists in enhancing engine acceleration with a given size propeller area.
In accordance with the various aspects of the invention, a marine propulsion device includes a drive housing to which is attached a suitable engine, the exhaust of which is pumped downwardly through a suitable passage in the drive housing to adjacent a torpedo housing carrying at least one propeller designed for surface running. A generally horizontal antiventilation plate is disposed above the torpedo housing, and a strut extends between the plate and the torpedo housing, just forwardly of the upper portion of the propeller.
Broadly, substantially all of the engine exhaust is forced by the engine from the drive housing passage for discharge into the path of the upper portion of the propeller. In one embodiment, substantially all of the exhaust passes through the strut and is discharged rearwardly therefrom into ventilating engagement with the forward face of the propeller. In another embodiment, a portion of the exhaust is also discharged downwardly through the antiventilation plate onto the upper edge portion of the propeller.